Modern air separation unit (ASU) plants typically use adsorbent beds to remove impurities in feed air before cryogenic distillation takes place. These pre-purification units (PPUs) typically remove components from air that can be detrimental to proper cryogenic air separation. These impurities include water, carbon dioxide, carbon monoxide, hydrogen and hydrocarbons such as butane, propylene and acetylene. Other hydrocarbons such as propane and ethylene, and nitrous oxide are partially removed in PPUs. The removal of these impurities improves process safety of the ASU and eliminates the need for low temperature absorbers.
Feed air is purified by adsorption of impurities onto the surface of adsorbents such as activated alumina and molecular sieves that are contained in the PPU vessel. Oxygen, nitrogen and argon are not adsorbed and pass through the vessels. PPU vessels have a finite capacity for adsorption and must be regenerated so there are usually two or three beds on alternate duty. When one PPU bed reaches its saturation point, the air is switched to flow through the other bed. During regeneration, PPUs are depressurized and regeneration gas is passed through the off-stream vessel to remove the adsorbed components. As discussed later, FIG. 1 describes a typical two bed, dual-layer PPU arrangement.
The adsorption beds in the PPUs may contain more than two layers to remove other impurities in the feed air. For example, a palladium and a carulite layer are usually added in the adsorbent bed to remove hydrogen and carbon monoxide by converting them to water and carbon dioxide respectively before the feed stream enters the cryogenic distillation.
The present inventors have discovered that the use of ionic liquids in the pre-purification of air prior to cryogenic distillation offers a number of advantages over the use of traditional adsorbents such as activated alumina and molecular sieves. These advantages include the ability to remove impurities in feed air more effectively to achieve continuous operation thus allowing for simpler process designs; more effective catalyst-product separation; reduction of vessel volume and associated costs; reduction in energy consumed and its commensurate costs; and reduction in malfunctions encountered in adsorbent beds due to the damage of adsorbents caused by high velocity gas flows and rapid pressure changes.